1. Field of the invention
This invention relates to a liquid sealed cylindrical-type vibration preventing apparatus suitably used for an engine mount of a vehicle, for example.
2. Related Art
Conventionally, when an engine of a vehicle which generates a vibration is mounted onto a body frame, an engine mount which supports the engine for preventing or absorbing the vibration has been used. One of such liquid sealed cylindrical-type vibration preventing apparatus is disclosed in Japanese Patent Laid-open (Koukai) No. 1-176827 and is shown in FIGS. 7 and 8.
This liquid sealed cylindrical-type vibration preventing apparatus is comprised of a pipe-shaped main-shaft metal fitting 101, a generally cylindrical rubber elastic (resilient) body 103, a first diaphragm 104 and a second diaphragm 107, an annular orifice forming member 105, and an outer cylindrical metal fitting 106.
In detail, the rubber elastic body 103 is attached outside the main-shaft metal fitting 101 and has a pair of opened recesses 135 and 136 on an outer surface thereof. The first and second diaphragms 104 and 107 are formed integral with the rubber elastic body 103 and is disposed in one recess 135 circumferentially. The orifice forming member 105 is mounted outside the rubber elastic body 103 and covers opened portions of each recess 135 or 136 so that a main liquid chamber 155 in which a liquid (L) is filled and sealed is formed in another recess 136, and a first sub liquid chamber 156a and a second sub liquid chamber 156b are formed between the orifice forming member 105 and the first and second diaphragms 104 and 107. The orifice forming member 105 is also formed, at an outer peripheral surface thereof, a first orifice passage 157 which communicates the main liquid chamber 155 with the first sub liquid chamber 156a and a second orifice passage 158 which communicates the main liquid chamber 155 with the second sub liquid chamber 156b. The outer metal fitting 106 is coaxially disposed outside the orifice forming member 105.
This liquid sealed cylindrical-type vibration preventing apparatus is mounted onto the vehicle, by attaching the main-shaft metal fitting 101 to a mount portion of one of the vehicle body side and engine side by bolts etc., and by engaging the outer-cylinder metal fitting 101 under pressure to another mount portion. Upon the mounting, the vibration preventing apparatus is directed so that the main liquid chamber 155, is located at an upward portion or a downward portion through which the main vibration of the engine is transmitted.
When the vibration orthogonal to the main-shaft metal fitting 101 is applied between the main-shaft metal fitting 101 and the outer cylinder metal fitting 106 in this condition, the volume of the main liquid chamber 155 varies due to the elastic deformation of the rubber elastic body 103, so that the liquid (L) flows between the main liquid chamber 156 and the first and second sub liquid chambers 156a and 156b via the first and second orifice passages 157 and 158. Upon the liquid-flowing, the vibration of the engine shake (about 10 Hz) can be effectively reduced by a liquid pillar resonance operation of the liquid (L) which flows in the first orifice passage 157. Also, the idle vibration (about 25 Hz) can be effectively reduced by a liquid pillar resonance operation of the liquid (L) which flows in the second orifice passage 158.
Thus, this vibration preventing apparatus can reduce the two kinds of vibration of different, relatively small frequencies and of which difference is relatively small, by the liquid pillar resonance operation by the liquid (L) which flows in the first and the second orifice passages 157 and 158.
By the way, to obtain the aimed frequency for tuning by selecting the cross-section area and the passage length of the first and second orifice passages 157 and 158, and the spring rigidity of the first and second diaphragms 104 and 107, the rubber elastic body 103 and the orifice forming member 105 which can be adapted to such tuning specification should be prepared and assembled. Here, the resonance frequency can be set higher when the cross-section area of the first and second orifice passages 157 and 158 is selected larger, the passage-length of the first and second orifice passages 157 and 158 is selected shorter and the spring rigidity of the first and second diaphragms 104 and 107 is selected higher or larger. Accordingly, the frequency is adjusted by preparing the rubber elastic body 103 and the orifice forming member 105 having the construction adapted to the required tuning specification and assembling them.
However, the recent diversification of the required specification needs various kinds of the construction members. Preparing the construction members for each specification is disadvantageous in the manufacturing cost and the administration.
In addition, there is a case where the idle vibration of about 25 Hz and a low-speed confine noise of about 80 to 100 Hz should be reduced, instead of the engine shakes (10 Hz) and the idle vibration (20 Hz). It is noted that the low-speed confine noise is the vibration which has the relatively higher frequency different from that of the idle vibration. For this reason, the conventional liquid sealed cylindrical-type vibration preventing apparatus which has the above mentioned construction can be hardly turned to reduce the idle vibration and the low-speed confine noise.